Sunday, May 8, 2011

Mission

Since I'm doing all the design on this bike, the number of choices is a little overwhelming. To clarify things, I decided to set the mission I'm trying to accomplish. This is a big thing in the private pilot world because everyone wants the big fast plane, but realistically you are better off choosing your mission and buying a plane to fit it.

My primary goal with the bike is to get to and from work. While I can take a bus, the 28 local is routed through downtown and past the stadiums. This means if there is an event at the stadium, or just regular friday traffic, I can spend over an hour waiting for 3 busses to come at once. I hate at-grade public transport. I grew up outside of Boston (Newton) riding the T which is mostly separate from traffic and it set my standard for efficiency. San Francisco was similar, but Seattle really is the pits.

My work commute isn't very far: just about 3 miles. It's all downhill in the morning, and has about 1 mile of uphill on the way home. There is a stoplight at 8th and 65th which is on a 6% grade.

  • about 10 miles with some hills on a single charge with no pedaling (gives me errands in ballard)
  • max speed at 20 mph
  • Need enough power for the stop light on the 6% grade
  • Also have a stop on a 4% grade at NW Leary and 8th.

One key thing here is my decision to follow the law about max speed. Lots of ebike people want FAST. I've ridden motorcycles on and off since I was 16 (that's 21 years if you are counting) and I fucking love them. But with increasing speed comes increasing kinetic energy (increases at the square of the speed, so a change from 20mph to 30mph increases your energy, which means more injuries and a need for better protective gear. I don't really want to break out the Roadcrafter and motorcycle helmet for my bicycle ride to work. I'll go buy a motorcycle if I want that.

Math geeks need apply:

The kinetic energy stored in my body at 20mph is 3200 joules. 
At 30mph it becomes 7200 joules. 
Therefore the speed increase of 10mph adds 4000 joules of stored energy into me that needs to get dissappated if I smack into something, or turned into heat by my brakes so I can stop.

By limiting my top speed I gain the following:
  • smaller brakes
  • smaller safety gear (with a similar margin of safety)
  • better handling (the steering geometry of my comfort bike is not designed for 30mph)
  • cheaper powertrain

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